Perfluoro compounds are excellent in chemical stability, dielectric constant lowness, refractive index lowness, viscosity lowness, lubricity, water repellency and oil repellency, non-adhesiveness, and the like, and they are applied to various materials, such as fluorocarbon rubbers, materials for plenum cables, optical fibers, antireflective films, pellicles, and the like. In recent years, therefore, methods of synthesizing a perfluoro compound effectively have actively been developed.
One method of synthesizing a perfluoro compound effectively is a direct fluorination method using fluorine gas. This method is very attractive from the viewpoint of energy-saving and high selectivity of a product, since, for example, this method is different from electrolytic fluorination, which requires vast energy and may generate a compound composed of mixed isomers. However, the direct fluorination method has a problem that the following occasionally advance: cleavage of carbon-carbon chains, oligomerization, generation of high heat, and explosive reaction.
Known methods of solving such a problem are to inject a substrate into an inert solvent, together with an excessive amount of fluorine (see, for example, U.S. Pat. Nos. 5,322,904 and 5,399,718, and International Publication No. WO 00/56694 pamphlet). In this method, fluorine is saturated into a solvent inert to fluorine, which will be referred to as an inert solvent hereinafter, and examples thereof include flon-series solvents and perfluoro alkane-series solvents. A substrate and fluorine are then introduced into the inert solvent such that the amount of fluorine would be excessive, i.e. the mol number of fluorine is not less than the amount necessary to fluorinate the substrate completely, thereby restraining coupling reaction between molecules of the substrate, decomposition reaction of the substrate, and explosive reaction. However, if the substrate accumulates in a reaction container without being effectively fluorinated and the reaction proceeds all at once at some point of time, it is expected that there is risk due to much heat generation and explosion is possible.
To avoid such risk, it is indispensable to cause the fluorination to proceed effectively from the early stage of the reaction. For this purpose, it is necessary to set conditions for causing the substrate to react effectively with fluorine.
One condition for causing the substrate to react effectively with fluorine is that “the substrate is soluble in a solvent and has a bond that reacts easily with fluorine.” Of inert solvents, flon-series solvents are excellent in dissolving power. Thus, fluorinating reaction using a flon solvent is known (see, for example, U.S. Pat. Nos. 5,322,904 and 5,399,718). However, flon-series solvents have the problems that they are not easily available and that they are environmentally harmful compounds that deplete the ozone layer. Consequently, it would be difficult to use the flon-series solvents for the production of a perfluoro compound. Further, even in the case of using a flon-series solvent, there is the problem of explosion, depending on a structure of the substrate to be used (see, for example, Journal of Synthetic Organic Chemistry, Japan, 2003, vol. 61, No. 2, p. 164).
In another known example, a solvent other than flon-series solvents, such as Fluorinert FC-72 (trade name, manufactured by 3M Co.), is used to conduct fluorination (see, for example, U.S. Pat. No. 5,399,718). This solvent is inferior to flon-series solvents in power for dissolving hydrocarbon compounds. When this solvent is used to perform fluorination, the reaction proceeds as in the example described in U.S. Pat. No. 5,399,718, if a substrate may be dissolved in the solvent even in a small amount thereof, and the substrate has a bond that reacts easily with fluorine. However, if the substrate is hardly dissolved in the solvent, and further the substrate has a bond that does not react easily with fluorine, it is observed that the reaction thereof does not proceed at all, and the substrate floats in the reaction system.
When the solubility of a substrate in inert solvent is poor, another known method is to perform fluorination using a compound in which fluorine is partially introduced into the substrate in advance (see, for example, International Publication No. WO 00/56694 pamphlet, and Journal of Synthetic Organic Chemistry, Japan, 2003, vol. 61, No. 2, p. 164). However, in this case, the substrate is restricted to a substrate into which fluorine can be introduced in advance, thereby causing a problem that the range of compounds that can be fluorinated becomes narrow. Further, there is caused a problem that the cost for the substrate's raw material increases, since fluorine is beforehand introduced into the material.